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Unusual Behavior in GEO: SJ-17

This data repository accompanies Space Threat Assessment, a report series from the CSIS Aerospace Security Project.

This data repository visualizes the orbital position of Chinese satellite SJ-17 from January to December 2020.

Unlike most objects in the geostationary belt, SJ-17 made a series of orbital maneuvers after it reached its destination orbital regime, varying its position relative to the Earth and neighboring satellites. In 2018, SJ-17 occupied a wide span of positions on orbit, from 37.7°E (corresponding to an orbital position overlooking eastern Africa) to 180°E (corresponding to an area overlooking the U.S. Marshall Islands).

In early 2018, defense news outlet Breaking Defense published a detailed list of SJ-17’s movements—provided by Analytical Graphics, Inc., an engineering software company—including a list of satellites that may have engaged directly with it through remote proximity operations (RPO).1 The table below describes a more complete history of SJ-17’s orbital history, including several of the satellite’s nearest neighbors.

Learn more about the potential consequences of such behavior on orbit, including a broader look at China’s counterspace weapon activities in this year’s edition of Space Threat Assessment.


This data visualization relies on one principal data source: the catalog of all space objects, provided by the U.S. Air Force’s 18th Space Control Squadron.3

The orbital position data for SJ-17 from July 2017 to December 2020—shown in orange in the interactive diagram—was derived from the two-line element (TLE) data for the satellite, available at The TLE for a space object is a measurement of the object’s approximate orbit (its inclination, right ascension of the ascending node, eccentricity, and argument of perigee) and its position on that orbit (its mean anomaly). This data was transformed into a time-dependent longitude position using PyEphem, a publicly-available Python package for high-precision astronomy computations.4 Although often provides more than one TLE for SJ-17 per day during the time period depicted, this data repository shows just one longitudinal position per day, for clarity. Longitude measurements are propagated to midnight on each day.

To identify SJ-17’s neighboring satellites, the longitude transformation process was repeated for all satellites in the geosynchronous region (defined as geostationary altitude plus or minus 200 km, with inclinations between -15 and 15 degrees) from SJ-17’s launch in November 2016 until December 2020.5 Satellites that sustained the closest longitude measurement to SJ-17 for a period of time are known as the satellite’s nearest neighbor. Satellites that remained SJ-17’s nearest neighbor for at least one week, coming closer than 0.075° (approximately 55 km) have been included in the above table, in addition to those neighbors identified by Breaking Defense.

How far apart are satellites in GEO during normal operations? The median distance between satellites, according to an analysis of all GEO satellites included in the catalog, is approximately 0.28° or about 207 km. Since not all satellites are included in the U.S. Air Force’s space object database—the 18th Space Control Squadron does not provide orbital data for classified U.S. satellites, for example—this methodology likely over-approximates separation distances.

About 25% of GEO satellites are separated by less than 0.11° (or about 80 km), while 75% of GEO satellites are separated by less than 0.69° (or about 508 km).

Most satellites—about 90%—are separated from their neighbors by more than 25 km. Similarly, about 96% of satellites are separated from their neighbors by more than 10 km.


This interactive data repository is a product of the Andreas C. Dracopoulos iDeas Lab, the in-house digital, multimedia, and design agency at the Center for Strategic and International Studies.

Special thanks to Jacque Schrag for her work developing this tool.